A cyanide process which employs cyanide as a complexing agent has been used exclusively for many years in, for example, hydrometallurgical recovery of gold from auriferous ores. However, the serious impacts of cyanide toxicity upon waste disposal and upon the environment have made it urgent to reconsider the process.
It has been proposed to use thiourea, sodium thiosulfate, or the like in place of cyanide as the gold complexing agent. These substitutes, however, if effective at all, make the treatment so much more costly that they have seldom come into practical use.
Chlorine was tried earlier, but its adoption was given up because of its strong corrosive attack and the consequent high treatment cost involved.
Lixiviating a gold-containing material with iodine to recover gold is well known in the art. For example, processes for recovering gold by lixiviation with an iodine/iodide solution are described in U.S. Patent Nos. 2,304,823, 3,957,505, and 4,557,759. These processes entail significant loss of expensive iodine and are not economically warranted.
A process for efficiently achieving both electrolytic recovery of gold and regeneration of iodine from gold-containing iodine solution leached with an aqueous solution containing elemental iodine and iodide ions, taking advantage of the oxidizing power of iodine and the gold-complexing action of the iodide ion, has been established. For details, refer to PCT Patent Application Publication No. 502358/1988 (International Publication No. WO87/03623). The process (hereinafter called the "iodine process") may be defined as: "A process for recovering gold by electrolysis from a pregnant, gold-bearing iodine lixiviant in which gold has been leached from a gold-containing material with an iodine/iodide lixiviant, while, at the same time, oxidizing a portion of the iodide ions in the lixiviant to regenerate iodine and recycle the lixiviant to the gold-leaching step."
To be more specific, the process comprises the steps of introducing the gold-bearing iodine lixiviant into the cathode compartment of an electrolytic cell, where gold is electrodeposited on the cathode, reducing iodine in the lixiviant substantially to iodide, and conducting the effluent solution from the cathode compartment into the anode compartment, where the iodide ions are oxidized for regeneration to elemental iodine.
The iodine process is attracting attention as an excellent method for gold recovery taking the place of the cyanide process and offers the following advantages:
(1) It has fewer deleterious effects upon the environment;
(2) Iodine in the lixiviant solution is stable in the form of a complex salt (I.sub.3.sup.-), and therefore, iodine loss during handling is minimized and the iodine concentration is easy to control;
(3) The resulting gold complex salt is highly stable; and
(4) Regeneration of iodine permits recycling of the spent lixiviant solution, realizing low cost operation.
Gold-bearing materials normally contain substances other than gold which are leachable with iodine. For example, in auriferous ores, ferrous minerals such as pyrite and pyrrhotite usually occur. Even in scraps containing precious metals it is common that iron, copper, and the like, are present. When such a gold-containing material is leached with an iodine lixiviant solution, the proportion of the resulting heavy metal ions to the gold ions in the pregnant lixiviant is generally about equivalent to or several figures larger than the latter. It has been recognized that the pregnant lixiviant, if subjected without prior purification to simultaneous electrolytic recovery of gold and iodine regeneration for reuse of the lixiviant, would present the following problems:
(a) At the cathode of the electrolytic cell, electrolysis of part of the water takes place, making the cathode solution alkaline. Consequently, hydroxides of heavy metals other than gold are formed in the cathode compartment, and they interfere with the gold deposition onto the cathode.
(b) When the regenerated electrolyte, still containing the heavy metals, is reused in leaching gold from a fresh feed of gold-containing material, a substance unleachable with the iodine lixiviant solution tends to form on the surface of in the gold-containing material. If occurs, the leaching of gold could be hampered due to this passivation of the gold surface.
For wider use of the iodine process, the solution to these problems is imperative. In particular, it appears that there will be a growing requirement in the future for broadening the range of gold-containing materials to which gold recovery by the iodine process is applicable. To meet the requirement, it is important to eliminate the troubles that would arise from the presence of iron and other heavy metals.
In order to solve these problems, the present inventor has studied the following means:
(1) Adding a buffering agent such as potassium carbonate or sodium acetate to the iodine lixiviant solution in advance, to prevent the heavy metals in the gold-containing material, other than gold, from coming into solution.
(2) Forming precipitates of heavy metal hydroxides by the addition of an alkali the pregnant lixiviant containing heavy metals besides gold, and removing the precipitates from the lixiviant before being sent to the electrolysis step.
(3) Identifying an ion-exchange capable of selectively adsorbing heavy metals other than gold while allowing gold and iodine to pass therethrough, and then causing the pregnant lixiviant containing heavy metals besides gold to flow through the resin so that the heavy metals are removed before lixiviant is conducted to the electrolysis step.
Investigations by the present inventor have shown that approach (3) was the best means to solve the aforementioned problems with respect to feasibility. The approach (1) prevents the leaching of heavy metals other than gold to some extent but can hardly achieve perfect prevention, because the non-gold heavy metals occur in larger amounts than gold in the gold-containing materials. If the buffering agent is used in an amount large enough to avoid the leaching of the heavy metals, it also interferes with the leaching of gold, hampering the attainment of the objective of the process, i.e., the recovery of gold with the aid of iodine. Although heavy metals can be removed by approach (2), part of the gold-iodine complex precipitates together with hydroxides of the heavy metals. This necessitates the provision of an extra process step for the recovery of gold that has coprecipitated with the heavy metals, adding to the complexity of the process for gold recovery with iodine. Moreover, because the rates at which the precipitates are formed and are removed after concentration are much lower than the rates of gold leaching and electrolysis, the removal step slows down the overall speed of the iodine process for gold recovery.
In contrast to these two, the approach (3) makes it possible to remove heavy metals with substantially no deleterious effect upon the gold recovery process, provided an ideal ion-exchange resin for the purposes of the invention can be found.
Extensive investigations by the present inventor have now resulted in successful discovery of a cation-exchange resin which can pass gold and iodine, but selectively intercept and remove Fe ion and which lends itself to gold recovery by the iodine process. Styrenic, strongly acidic (strong-acid) cation-exchange resin meets the requirements. The styrenic, strongly acidic cation-exchange resin is commercially available, but has not previously been used for the above purpose.